Conventionally, articulated robot arm mechanisms are used in an industrial robot and various other fields. Such articulated robot arm mechanisms are combined and equipped, for example, with linear extension and retraction joints. An arm section constituting the linear extension and retraction joint is constituted by overlapping two types of connection piece strings in which a plurality of pieces each having the same shape are connected in a string, for example. By overlapping the two types of connection piece strings, a rigid state is formed, and a columnar body having a certain degree of rigidity is constituted. When a motor of the linear extension and retraction joint is rotated forward, the arm section in the shape of the columnar body is sent out from an ejection unit, and when it is rotated backward, the arm section is drawn back. The overlapping state of the connection piece strings is released at the rear of the ejection unit, the rigid state is returned to a bent state, and the connection piece strings are housed in a main body part in that state.
Adoption of this linear extension and retraction joint in an articulated robot arm mechanism makes an elbow joint unnecessary, and since a singular point can be easily eliminated, this linear extension and retraction joint will be an extremely useful structure from now on. The important matters with respect to the linear extension and retraction joint are to improve rigidity in the rigid state by overlapping the two connection piece strings, and also to realize smooth operations when sending-out and pulling-back the connection piece strings to the front and rear. In addition, when two connection pieces are linearly aligned and are sent out to the front while overlapped, it is also important to reduce a collision sound generated a collision of the two connection piece strings and a collision of connection pieces, particularly in a situation in which the robot in question is operating in the vicinity of a worker.